passivation of pressure sensitive adhesive stickies...
TRANSCRIPT
Author version
Passivation of pressure sensitive adhesive stickies by addition of acrylic fibers to OCC pulp
before papermaking
Mohammad Hadi Aryaie Monfared, Hossein Resalati, Ali Ghasemian and Martin A. Hubbe
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ABOUT THE AUTHORS
Stickies present in suspensions of recovered fibers cause a variety of operational problems and
are difficult to remove from the pulp. Due to their viscoelastic nature, they generally can’t be
screened from the pulp with high efficiency. That’s why, in the present work, we looked for a
way to passivate stickies by allowing them to become bound to plastic fibers.
The present results, at the laboratory scale, suggest two possible paths toward
implementation. On the one hand, a fiber recovery process could be optimized such as to remove
higher levels of stickies from the pulp prior to papermaking. But the present results suggest some
advantages to allowing the plastic fibers, along with their load of passivated stickies, to become
incorporated into the paper product, where they can increase pulp freeness and contribute to
paper strength and bulk.
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Mohammad Hadi Aryaie Monfared is a faculty member at Gorgan University of Agricultural
Sciences and Natural Resources, Gorgan, Iran. Hossein Resalati is a faculty member at Sari
University of Agricultural Sciences and Natural Resources, Sari, Iran. Ali Ghasemian is a faculty
member at Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran.
Martin Hubbe is a faculty member at North Carolina State University, Raleigh, NC, USA.
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ABSTRACT: This study investigated the addition of acrylic fiber to Old Corrugated
Container (OCC) pulp as a possible means of overcoming adverse effects of water-based
pressure sensitive adhesives during manufacture of paper or paperboard. Such
adhesives can constitute a main source of stickies, which hurt the efficiency of the
papermaking process and make tacky spots in the product. The highest amount of acrylic
Author version
fiber added to recycle pulps generally resulted in a 77% reduction in accepted pulp
microstickies. The addition of acrylic fibres also increased pulp freeness, tear index, burst
strength, and breaking length, though there was a reduction in screen yield. Hence, in
addition to controlling the adverse effects of stickies, the addition of acrylic fibers resulted
in the improvement of the mechanical properties of paper compared with a control
sample.
Application: These research results reveal possible ways to employ plastic fibers in
recovered fiber furnish as a means of overcoming problems with stickies. Not only can
the stickies be passivized, but paper properties can be improved as well.
Author version
Stickies and tacky substances are common in typical batches of recovered paper. They can
be a source of numerous problems such as reductions in the appearance properties of paper sheets
and lowering of the efficiency of paper machine operations caused by an increase in the frequency
of wash ups, web breaks and felt plugging [1]. Some of the most difficult problems result from the
presence of adhesives such as labels and adhesive tape. Pressure-sensitive adhesives (PSA), which
are often a key component of stickies, are organic copolymers and mixtures that may include
styrene-butadiene and styrene acrylic latex binders, rubber, vinyl acetates, polyisoperene,
polybutadiene, and hotmelts [3]. PSA materials are hard to remove in recycling, and they deposit
on equipment and are retained in the product, hampering operations [2]. Adequate management
and control of stickies is necessary for the economic health of the industry. There continues to be
considerable environmental pressure to increase the use of recycled paper, but its use is not without
complications [3].
Deposition of stickies on paper machine surfaces, wires, felts, fabrics, and rolls results in
problems such as wet end breaks, press and dryer sections breaks, holes, sheet defects, high dirt
counts, and difficulties in paper converting. These deposits and their associated problems cause a
significant amount of downtime and waste. The economic impact of stickies rises considerably
when the cost of downtime, chemicals costs, production losses, scrap, rejected materials, and
customer complaints are taken into consideration [4].
Many studies have focused on the control of stickies. It is clear, however, that no single
solution can be found for the many problems caused by stickies on paper and board machines that
use fiber [5]. General reviews of control techniques for stickies can be found in several published
articles, such as: [1,2 ,5-8].
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Physiochemical methods are used to remove or control stickies by the modification of the
surface properties of stickies either through adsorption of mineral particles or addition of other
chemicals or in combination in order to facilitate the removal or pacification of the stickies. These
methods include: flotation, washing, addition of solid materials, and treatment with surfactants and
polymers [9]. Dispersants, surfactants, and solvents are used to minimize stickies at various stages
of the recycling process. Other approaches use chemicals to agglomerate the stickies in order to
enhance their removal at the screening stage. Other chemical treatments aim to make the stickies
more hydrophilic to ease their removal at the washing stage. In fixation treatments, the stickies are
attached to the paper sheet using a water-soluble polymer, which changes the surface charge of the
stickies. In dispersing and fixation treatments, first a dispersant is added to decrease the size of the
stickies, and then a cationic polymer is applied in order to adhere the stickies onto the sheet. In
passivation, the use of talc, dispersants, solvents, and low molecular weight cationic polymers
makes the paper machine less susceptible to stickies [9]. Other researchers have focused on the
design of new PSAs with better recyclability properties [10].
The development and the use of a variety of man-made fibers have in recent decades
created a revolution in the textile industry [2,6]. It is possible that similar application of synthetic
fibers may eventually become widespread in the paper industry. Many specialty paper products
are currently made of synthetic fibers [11]. Another study revealed that an addition of 4 to 10% of
short-cut polyester staple fibers to bagasse and wheat straw pulp can significantly improve the
mechanical properties of the manufactured paper [12]. In certain applications, synthetic
polypropylene fibers have been used for scavenging inks and sticky particles from secondary fiber
stock. For example, Scav EX 235 is a product of this type that has been supplied by Hercules, Inc.
(now Solenis). These types of synthetic fibers have met with some limited success [13].
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Acrylic fibers, which have been widely available for many years [14] and now have a
global production capacity of about six billion pounds, are synthetic fibers whose polymeric chains
are formed by a succession of monomeric units composed of predominantly acrylonitrile. The U.S.
Federal Trade Commission defines fibers containing more than 85% acrylonitrile in weight as
acrylic fibers [14]. These fibers are highly resistant to water, chemicals, heat, and ultraviolet light,
and are available in staple or in slightly fibrillated form. They have been used in a number of
nonwoven products, leading to the provision of unique properties [15].
An older study reported that synthetic fibers can act as an effective adsorbent of stickies in
papermaking systems [16]. It was reported that the addition of synthetic fibers with a high surface
area at a lower level can wrap and pacify sticky particles and thus prevent agglomeration and
minimize deposits in the paper machine system. Although this method has been shown to be
effective, no recent reports indicating the application of the aforementioned method have been
presented [6]. The cost and the inability of most synthetic fibers to bond with natural cellulose
fibers are the two essential factors that discourage the application of synthetic fibers in
conventional papermaking [11].
The aim of this research is to investigate the effects of adding acrylic fibers for the control
of stickies during papermaking operations and as a way to enhance the properties of the produced
sheets. Differing from most previous studies, in this work the acrylic fibers were added directly
to the OCC fiber suspension, such as to enable the interaction between their surfaces and suspended
matter in the mixture, and a portion of those fibers were subsequently removed by screening of the
pulp. In a future implementation of this technology some of the acrylic fibers would likely be
rejected in a screening operation during upgrading of the recovered fiber slurry, thus providing an
opportunity for removal of stickies and other oleophilic matter from the system. The remainder of
Author version
the acrylic fibers would then proceed forward through the papermaking process, where they would
presumably influence such properties as apparent density, tensile strength, and resistance to burst
stresses.
MATERIAL AND METHODS
Raw material Old corrugated container (OCC) pieces were randomly collected from post-consumer wastes. The
water-based acrylic PSA was purchased from a national company (Simab Resin). Some of its
specifications are shown in Table I.
Properties Amount (Qualitative & Quantitative)
Appearance Milky White Light
Type Non - Crosslinking
Emulsifying system Anionic - Nonionic
Solids (%) 59±1
pH(as packed) 6-8
Tg (°C) -35
Viscosity (cP) ≤5000
Mechanical Stability Good
I. Specifications of initial acrylic water base pressure sensitive adhesive (manufacturer brochure)
A rod coater at a rod speed of 70 mm/sec was used to apply a coating of PSA to a piece of silicon
paper in order to obtain a coating weighing 27 g/m2. This was then dried in an oven at 50 °C for 1
hour. The dried PSA resin film was pressed onto OCC sheet samples at 10% weight ratio.
The scavenger material, intended to collect and passivate oleophilic matter from the suspension,
consisted of industrial textile acrylic fibers purchased from Monte Fiber Hispania, Spain with the
trade name of Leacril® fiocco. The original fibers were cut to the selected length (approximately
2 mm) by scissors.
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Repulping
The prepared OCC material was presoaked in the water bath for 2 hour and then re-pulped
with 5 % consistency and at the 50 °C and 10000 round disintegration. The resulting suspension
was used in the subsequent steps.
Microstickies
Microstickies were measured in accordance with the Doshi method using the locally made
apparatus shown in Fig. 1 [17]. In this method two parallel stainless steel paddles hold two 12.7
cm by 2.54 cm High Density Polyethylene (HDPE) films rotating at a speed of 100 revolutions per
minute inside 12 liters of drained water containing microstickies at 65°C for 30 min. At the end of
this period the HDPE was removed from the frame and rinsed with deionized water with a
temperature of 20 °C, dried to a constant weight at 105 °C, and finally weighed.
1. Microstickies deposition tester and its propellers.
Turbidity measurement The filtrate collected from slot screening of OCC pulp was left to settle for 24 hours, and its
turbidity was then measured using a Wagtech turbidity meter in accordance with the T820-110
standard (Wagtech Company, UK) [3,4].
Macrostickies
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Macrostickies were measured according to TAPPI standard T 277 pm-99. The bone-dry pad of
macrostickies was weighed, and the contents were reported as percent of macrostickies.
Image analysis of macrostickies
Figure 2 shows photographs of the stickies on their filter pads, at various stages of processing. The
scanned image of the macrostickies pad at the highest resolution (600 dpi) of an LIDE 200 Canon
scanner was then analyzed with Image J software according to the processing steps shown in Fig.
3.
2. The various stages in the processing of the scanned image of the macro stickies padin Image J: A- Initial pad scan, B- Dyed pad, C- Binary of Image, D- Threshold image, E- Inversion of images, F- Clear outsides, G- Closed particle environment, H- Filled holes (final image for analysis)
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Fig 3. Flow chart of image analyzing method with Image J
The properties of handsheets
Handsheets were prepared according to TAPPI T205 sp-02 with a basis weight of 130±5 g/m2, and
their physical, optical, and mechanical properties tested according to the following TAPPI test
standards: Conditioning (T 402 sp-03), Brightness (T 452 om-02), Caliper thickness (T 411 om-
05), Tensile strength (T 494 om-01), Burst strength (T 403 om-02), Tear strength (T 414 om-04),
and Air Resistance (T 460 om-02).
SEM Images
Scanning electron microscopy (SEM) images were prepared using a LEO 1450 VP microscope,
Germany, with resolution 2.5 nm equipped with EDS sensor model 7353. The samples coated with
gold and palladium before imaging.
Statistical analysis
The results were analyzed with the standard pairwise t-tests. To simplify the comparison of means,
error bars with 95% confidence intervals are provided in all the figures. The following formula
and the corresponding t table were used for this purpose [18& 19].
Error Bar (95% Confidence Interval) =M ± (t (n–1) × SE)
SE: Standard Error M: mean n: sample size
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RESULTS AND DISCUSSION
Macro stickies
Figure 4 shows the impact of acrylic fiber on the relative weight of the macrostickies. The use of
acrylic fibers up to 1.5% led to an increase in the weight of macrostickies. However, application
of higher levels of acrylic fiber was not deemed to be significantly different relative to the 1/5 ratio
of acrylic fiber at the 95% confidence level. After attachment to the stickies, elongated acrylic
fibers seemed to result in formation of larger particles of stickies and their agglomeration. The
inability of natural and synthetic fibers to pass through Somerville screen slots resulted in the
increase in the weight of the collected macrostickies.
Fig 4. Impact of acrylic fiber on the weight of screened macro stickies with different treatment
dosages
Image analyses of macrostickies
Table II summarizes results for the analysis of macrostickes, as affected by optional addition of
acrylic fibers followed by screening. As the results indicate, addition of acrylic fibers was directly
0
5
10
15
20
25
30
35
0 0.5 1 1.5 2 2.5
Mac
ro S
tick
ies
OD
Bas
e W
eigh
t (%
)
Acrylic Fiber (%)
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related to the number, area, and logarithmic area density of the stickies. Addition of different levels
of acrylic fiber maintained a slight increase in the total number and area of the macrostickies.
When compared with the control sample, significant change was noticeable in the number and area
of stickies within different class sizes. In samples treated with acrylic fibers, an increase in fiber
content was detected despite the obvious reduction in the number of macrostickies. The surface
area of stickies and their calculated logarithmic area density rose when compared to the control
samples.
When acrylic fibers were added to furnish, the number and area of macrostickies in the
first two class sizes were considerably reduced, whereas an increase in the same parameters was
prevalent in the last four class sizes. The trend of change in fact accrued with class size, i.e. an
increase in the addition of acrylic fibers to pulp resulted in the reduction in the number of stickies
in the 100-200 and 200-300 class sizes, while the opposite occurred in the other class sizes. It
seems that the adherence of acrylic fibers to the PSAs prevented further dispersion and reduced
the production of smaller sticky particles in the suspension.
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Class Size
Diameter Range of stickies
(µm)
Control Acrylic Fiber 1.0 Acrylic Fiber 1.5 Acrylic Fiber 2.5
C A D C A D C A D C A D
1 100 - 200 51 6.6 2304.1 28 3.37 1164.16 24 3.92 1354.16 25 4.50 1582.16
2 200 - 300 36 8.6 2481.4 29 6.17 1776.18 32 8.16 2349.06 26 6.75 1943.15
3 300 - 400 18 6.6 1786.7 13 4.66 1252.06 16 5.03 1351.47 15 4.63 1244
4 402 - 500 21 9.5 2466.5 29 13.01 3370.72 31 14.09 3650.54 34 15.39 3987.35
5 500 - 600 20 11.0 2804.3 23 10.79 2733.43 25 13.84 3506.08 28 14.48 3668.21
6 600 - 700 27 17.0 4258.0 21 13.33 3325.72 23 14.98 3737.38 24 15.24 3802.25
7 700 - 1000 22 16.2 4538.7 28 21.19 5925.78 28 23.66 6616.51 32 23.76 6644.48
8 1000 - 2000
19 23.3 10061.2 32 34.81 15031.39 34 38.23 16508.19 39 42.77 18468.62
9 2000 - 5000
16 41.1 36854.3 26 57.40 51417.03 29 64,11 57417.62 31 71.98 64466.08
10 >5000 14 89.9 51777.1 25 147.82 85107.36 27 151.55 87254.91 29 152.60 87859.45
244 229.8 119332.3 254 312.55 171103.8 269 273.46 183745.9 283 352.1 193665.8
II. Complete summary of Image J analysis of macro stickies
C= Number of stickies measured A= Measured area of stickies (mm2) D= Logarithmic area density (mm2)
13
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Microstickies
The measurement of microstickies was based on a laboratory arrangement that aimed to simulate
the real deposition phenomena that occur in wet or dry sections of the paper machine [4]. Figure
5 shows microstickies properties at different treatment dosages of acrylic fibers. These results
indicated that the addition of synthetic fibers was the main reason for the reduction of deposited
microstickies and that stickies were attached to the surfaces of the synthetic fibers. It has been
suggested that the sticky substances can be adsorbed or wrapped by acrylic fibers, thus reducing
their tackiness and making them easier to remain in a paper sheet [20]. Therefore, the free tacky
particles, i.e. the amount of adhered particles to the deposition tester paddles, decreased, as shown
in Fig. 5.
Figure 5 also gives results from the water turbidity analysis of the filtrate that passed
through a slotted screen. The results show a substantial turbidity decrease with up to 1% addition
of acrylic fibers. It seems that the use of up to 1.5% acrylic fiber resulted in maximum collection
of microstickies so that there were less suspended particles in the drained water. Increasing the use
of acrylic fibers from 2% did not have a considerable effect. Use of a higher percentage of acrylic
fiber was not effective for the further reduction of micro sickies.
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Fig. 5. Microstickies content and turbidity of filtrate versus acrylic fiber addition level
Screen-accepted pulp amounts using different acrylic fiber addition levels are shown in Fig. 6.
According to these results, an increase in acrylic fibers resulted in the reduction of the acceptable
amount of pulp. In fact, the attachment of stickies particles to acrylic fibers yielded larger objects
(acting like macrostickies) that could not pass through screen slots. As a result, the amounts
rejected, or the increase in macrostickies, can explain the decrease in the final screen acceptable
pulp amount.
-2
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Mic
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tick
ies(
mlg
)
Tu
rbid
ity(N
TU
)
Acrylic Fibers (%)
Turbidity
micro stickies
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Fig. 6. Screen accept properties of treated pulp
The freeness values of recycled pulp with added acrylic fibers are shown in Fig. 7. As can be seen,
the addition of acrylic fibers increased the freeness of the obtained furnish. One way to account
for such an effect is that the acrylic fibers have no natural affinity for water absorption.
Furthermore, they significantly improved pulp freeness by the agglomeration of microstickies,
which otherwise might tend to clog channels in the wet web of paper during dewatering [21]. The
acrylic fibers also can be expected to provide a bulkier, more porous wet mat of fibers during
formation of the sheet. All three of these factors can be expected to favor more rapid release of
water from the wet web.
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86
0 0.5 1 1.5 2 2.5
Scre
en A
ccep
t (%
)
Acrylic Fiber (%)
Acrylic Fiber Blank
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Fig. 7. Freeness content of treated pulp
A change in bulk (the reciprocal of apparent density) with the addition of acrylic fibers can be seen
in Fig. 8. It can be seen that the bulk of handsheets increased from 1.79 for the control condition
to 1.98 cm3/g (10.61%) at an addition level of 2% acrylic fiber. One of the important reasons for
this increase is the weaker bonding between acrylic and natural fibers in comparison to the bonding
between two natural recycled fibers. Another reason for this is the non-collapsible nature of
synthetic fibers, which tends to make the paper thicker. It can be seen in Fig. 6 that addition of
more than 0.5% (base on bone dry weight of pulp) of acrylic fibers significantly increased the bulk
of the final handsheets.
638
640
642
644
646
648
650
652
654
0 0.5 1 1.5 2 2.5
Fre
enes
s (C
SF
)
Acrylic Fiber (%)
Acrylic Fiber Blank
17
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Fig. 8. Bulk properties of handsheets
As can be seen in Fig. 9, the effect of acrylic fiber addition on the air resistance of handsheets was
not significant, i.e. 100 ml air passed through the handsheets in about 3 seconds. The treated
handsheets showed no significant changes when compared with the blank sample.
Fig. 9. The effects of acrylic fiber addition to pulp suspension on air resistance of handsheets
0
0.5
1
1.5
2
2.5
3
3.5
0 0.5 1 1.5 2 2.5
Bu
lk (
Cm
3 /gr
)
Acrylic Fiber (%)
Acrylic Fiber Blank
0
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1.5
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2.5
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Air
Res
ista
nce
(S
ec)
Acrylic Fiber (%)
Acrylic Fiber Blank
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Stickies particles embedded on the surface of acrylic fibers were clearly apparent in the SEM
images. Figure 10 clearly shows sticky particles adsorbed on the surface of the acrylic fibers
following agitation.
Fig. 10. Micrographs of PSA surface that was treated with acrylic fibers. An arrow shows where an acrylic fibers is partly covering a large flake of PSA present in the sheet
Figure 11 show EDS graphs of treated pressure sensitive adhesives with acrylic fibers. This
spectrum shows that the calcium carbonate and the gold coating used in preparation of the
sample for SEM imaging are the materials prevalent in the sample.
Fig. 11. Elemental analysis of particles on the treated PSA with acrylic fibers
19
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The effects of the addition of acrylic fibers to recycled fibers and the mechanical and physical
properties of paper were also investigated. Figure 12 shows the effect of acrylic fiber addition to
pulp suspension on the burst index of the handsheets. It can be seen that the addition of up to 1.5%
acrylic fibers led to an increase in the burst strength. The bonding between acrylic fibers and
natural fibers is weaker than the bonding between two natural fibers [11]. However, because of the
agglomeration of micro stickies by acrylic fibers and the consequent cleaning of natural fiber
surfaces, the extent of hydrogen bonding between the natural fibers will increase. Also, it is likely
that the relatively long synthetic fibers will contribute to greater strength. Results showed that
using greater amounts of acrylic fiber can improve the mechanical properties of paper such as burst
index and burst energy absorption in comparison with the control sample (Fig. 12 and 13).
Fig. 12. The effects of acrylic fiber addition on pulp suspension on the burst index of the
handsheets
1.15
1.2
1.25
1.3
1.35
1.4
1.45
1.5
0 0.5 1 1.5 2 2.5
Burs
t In
dx (
KP
am2
/g)
Acrylic Fiber (%)
Acrylic Fiber Blank
20
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Fig. 13. The effects of acrylic fiber addition on pulp suspension on burst energy absorption of the handsheets
Generally, addition of acrylic fiber to low quality OCC recycled fibers improves the tear strength
of paper [12]. The basic properties of acrylic fiber, such as greater length, strength, and stiffness,
are the most important reasons for improving the tear strength (Fig. 14). The synthetic fibers such
as acrylic fibers, among the natural ones, act as a reinforcing rods and improve the strength
properties of the resultant paper [11].
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235
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245
250
255
0 0.5 1 1.5 2 2.5
Burs
t E
ner
gy A
bso
rpti
on (
j/m
2)
Acrylic Fiber (%)
Acrylic Fiber Blank
21
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Fig. 14. The effects of acrylic fiber addition to pulp suspension on tear index of handsheets
Breaking length, as one of the most important properties of paper, depends principally on the
effective number of bonding sites (hydrogen bonds) available on the fiber surface and on the
strength of single fiber [22]. Therefore, resistance is improved by increasing the usage of acrylic
fibers. Figure 15 indicates the significant improvement in the breaking length of handsheets by the
addition of acrylic fibers to OCC pulp.
2
2.5
3
3.5
4
4.5
0 0.5 1 1.5 2 2.5
Tea
r In
dex
(m
Nm
2/g
)
Acrylic Fiber (%)
Acrylic Fiber Blank
22
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Fig. 15. Effects of acrylic fiber addition to pulp suspension on the breaking length of handsheets
Practical Implications
The results of this study, though showing promise, suggest the need for further experimentation
and mill trial work to determine the best implementation strategies. A key area of concern lies in
whether or not it will be advantageous to incorporate substantial quantities of plastic fibers in the
selected paper product. Though the present work suggests a range of possible benefits in terms of
strength properties, freeness gains during papermaking, and higher caliper at a given basis weight,
it will be important to examine the detailed effects of the synthetic fibers on paper product
performance in different end-uses. While it is reasonable to suppose that sticky particles adsorbed
onto the surfaces of synthetic fibers within a sheet of paper will be so small and well incorporated
into the sheet as to not cause problems during converting and usage of a paper product, this
assumption would need to be checked in each case.
An alternative approach to future implementation of the technology demonstrated in the
present work may be to optimize the system such that essentially all of the synthetic fibers are
800
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0 0.5 1 1.5 2 2.5
Bre
akin
g L
ength
(m
)
Acrylic Fiber (%)
Acrylic Fiber Blank
23
Author version
removed from the system by screening. The present work shows that addition of acrylic fibers to
the fiber furnish as a whole was a promising option, leading to effective collection of stickies from
the suspension. By prudent selection of the length and other properties of such fibers, it seems
likely that a system can be optimized to allow semi-quantitative removal of the synthetic fibers –
together with their load of adsorbed stickies – from the system. Depending on how the recovered
fiber processing operations is set up, it might also be possible to clarify the process water in a paper
recycling plant by mixing fiber-free process water with synthetic fibers, followed by screening, as
a means of purging stickies from the system. The relative value of such processing would need to
be weighed relative to the cost of materials – also noting that such an approach would not be
benefiting from any possible contribution of the synthetic fiber inclusion in the paper product.
CONCLUSIONS
This investigation used commercially available suspension of model PSA stickies in a laboratory
system to determine the adsorption, wrapping, and pacification of sticky particles onto acrylic
fibers. The result showed that the use of acrylic fibers can effectively change the macro- and
microstickies ratio in recycled pulp and improved freeness of treated pulp. Results confirmed that
the use of acrylic fibers for control of stickies can promote the mechanical strength properties of
the final handsheets. Thus, a case can be made for further work in which synthetic fibers are
incorporated into the processing of recovered fibers with the aim of achieving a combination of
benefits – the passivation of stickies, the possible increase of production rates, and the
improvement of key paper properties.
ACKNOWLEDGEMENTS
24
Author version
We gratefully acknowledge the financial support provided by the Iran National Science
Foundation (INSF) for the financial supports (Grant No. 90003557) and the Gorgan University of
agricultural sciences and natural resources, as Ph.D. thesis supports. We also thank Dr. Alireza
Shakeri for his kind advice in the different steps of this research.
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